Concepts Involved in the Optimization of the Greenhouse Environment for Crop Production

Greenhouse vegetable crop production is based on control of the environment in such a way as to provide the conditions that are most favorable for maximum yield. A plant's ability to grow and develop is dependent on the photosynthetic process. In the presence of light, the plant combines carbon dioxide and water to form sugars which are then utilized for growth and fruit production. Photosynthesis is practically the only mechanism of energy input into the living world (Salisbury and Ross, 1978). Optimization of the greenhouse environment is directed at optimizing the photosynthetic process in the plants, the plant's ability to utilize light at maximum efficiency.

Tied closely to the photosynthetic process is the process of transpiration. Transpiration can be defined as the evaporation of water from plants (Salisbury and Ross 1978). Transpiration occurs through pores in the leaf surface called stomata (Salisbury and Ross 1978, Papadakis et al 1994). As water is lost from the leaf, a pressure is built up that drives the roots to find additional water to compensate for the loss. The evaporation of the water from the leaf serves to cool the leaf ensuring that optimum leaf temperatures are maintained. As the roots bring additional water into the plant, they also bring in nutrients which are sent throughout the rest of the plant with the water. Water is a key component of photosynthesis, as is carbon dioxide (CO2) which is often the limiting component of the process. The plant's source of carbon dioxide is the atmosphere as carbon dioxide exists as a gas at temperatures in the growing environment. Carbon dioxide enters the plant through the stomata in the leaves, and this is where it can be seen why transpiration represents a compromise to photosynthesis for the plant.

Figure 3. Cross section of a leaf showing stomata.

Plants have control over whether the stomata are open or closed. They are closed at night and open in response to the increasing light intensity that comes with the morning sun. The plant begins to photosynthesize and the stomata open in order to allow more carbon dioxide into the leaf. As light intensity increases, so does leaf temperature, and water vapour is lost from the leaf which serves to cool the leaf. The compromise with photosynthesis occurs when the heat stress in the environment causes such a loss of water vapour through the stomata that the movement of carbon dioxide into the leaf is reduced. The other factor involved with this process is the relative humidity in the environment. The transpiration stress on a leaf, and the plant at any given temperature, is greater at a lower relative humidity than a higher relative humidity. There also comes a point where the transpiration stress on the plant is so great that the stomata close and photosynthesis stops completely.

Photosynthesis6 CO2 + 12 HO - Light energy -> C6H12O2 + 6 O2 + 6 H2O

Photosynthesis is the plant process by which carbon dioxide and water with the input of light energy yields sugars, oxygen and water.

Photosynthesis is one of the most significant life processes, all the organic matter in living things comes about through photosynthesis. The above formula is not quite complete, as photosynthesis will only take place in the presence of chlorophyll, certain enzymes and cofactors. Without discussing all these requirements in detail, let it be enough to say that these cofactors, enzymes, and chlorophyll will be present if the plant receives adequate nutrition. One other point to clarify is that it takes 673,000 calories of light energy to drive the equation (Wilsone and Loomis, 1967).

Photosynthesis requires certain inputs to get the desired outputs. Carbon dioxide and water are combined and modified to produce sugar. The sugars are further used to form more complex carbohydrates and oils and so on. Running along side of the photosynthetic process are many more processes in the plant which help ensure that the plant does whit it is designed for. From the grower's point of view, the results of photosynthesis is the production of fruit. This serves to remind that the management decisions made in growing crops affect the outcome of how well the plant is able to run its photosynthetic engines to manufacture those products which are shipped to market. Growers provide the nutrition and environment which direct the plant to optimize photosynthesis and fruit development. Crop management decisions require a knowledge of how to keep the plants in balance so that yield and productive life of the crop is maximized.

Respiration is another process tied closely to photosynthesis. All living cells respire continuously (Salisbury and Ross 1978), and the overall process involves the breakdown of sugars within the cells, resulting in the release of energy which is then used for growth (Wilson and Loomis 1967, Salisbury and Ross 1978, Tootil and Blackmore 1984). Through photosynthesis plants utilize light energy to form sugars, which are then broken down by the respiration process, releasing the energy required by plant cells for growth and development.

Photosynthesis responds instantaneously to changes in light (Seginer 1996) as light energy is the driving force behind the process (Salisbury and Ross 1978). Generally, light is a given, with greenhouse growers relying on natural light to grow their crops. Optimization of photosynthesis can occur through providing supplemental lighting when natural light is limiting. This strategy is not common with the economics involved in supplemental lighting being the determining factor. The common strategy for optimizing photosynthesis comes about through optimizing transpiration. If, under any given level of light, transpiration is optimized such that the maximum amount of carbon dioxide is able to enter the stomata, then photosynthesis is also optimized. The benefit of optimizing photosynthesis through controlling transpiration is that the optimization can occur over both low and high light levels, although photosynthesis is naturally lower under lower light levels. Supplemental lighting is only useful in optimizing photosynthesis at low light levels.

Inherent to high yielding greenhouse crop production are the concepts of plant balance and directed growth. A plant that is growing in the optimum environment for maximum photosynthetic efficiency may not be allocating the resultant production of sugars and starches for maximum fruit production. Greenhouse vegetable plants respond to a number of environmental triggers or cues, and can alter their growth habits as a result. The simplest example is whether they have a vegetative focus or a generative focus. A plant with a vegetative focus is primarily growing roots, stems and leaves, a plant with a generative focus is concentrating on flowers and fruit production. Vegetative and generative plant growth can be thought of two ends of a continuum, the point where maximum sustained fruit production takes place is where vegetative growth is balanced with generative growth. Complete optimization of the growing environment for crop production also includes providing the correct environmental cues to direct the growth of plants to maintain a plant balance for profitable production.

The critical environmental parameters affecting plant growth that growers can control in the greenhouse are temperature, relative humidity, carbon dioxide, nutrition, availability of water, and the growing media. The way environment affects plant growth is not necessarily straightforward and the effect of one parameter is moderated by the others (Stanghellini and Van Meurs 1992). The presence of the crop canopy also exerts considerable influence on the greenhouse environment (Hanan 1990). The ability of growers to provide the optimal environment for their crops improves over time with experience. There is a conviction that environmental control of greenhouses is an art which expert growers bring to perfection (Seginer 1996). This being said, there are basic rules, and environmental setpoints that beginner growers can follow as a blueprint to grow a successful crop.

As the plants develop from the seedling phase to maturity, the conditions which determine the optimum environment for the crop also change (Seginer and McClendon 1992). Further, even when the crop is into full production, modifications of the environment may be necessary to ensure that maximum production is maintained. For example, the plants may start to move out-of-balance to become too vegetative or too generative. Through all stages of the crop cycle, growers must train themselves to recognize the indicators displayed by the crop to determine what adjustments in the environment are necessary, if any.